There are a number of commercially available devices for indicating when material, particularly liquid material, has reached a level to fill a gap or slot between two ultrasonic piezoelectric transducers. One transducer is energized to transmit energy through the material space toward the other. When air fills the material space, the ultrasonic energy is attenuated before reaching the other transducer due to the relatively poor sonic conductivity of the air. On the other hand, when the space is filled with liquid, the ultrasonic energy reaches the second transducer, and is detected by appropriate electronics to indicate that material has reached the level of the transducers. Since absence of detected energy at the receiving transducer can result from failure of the measurement apparatus, including failure of either transducer or of the bond between either transducer and its associated window, it is desirable to provide a mechanism for testing system integrity and operability.
It is a general object of the present invention to provide an apparatus of the described character for determining material level that includes facility for automatic self-testing of the entire apparatus, including the transmitting and receiving transducers and bonding thereof to the associated windows. A more specific object of the present invention is to provide an apparatus of the described character that is economical to fabricate, that automatically performs the self-testing function during operation without operator intervention, that provides an indication of system failure to an operator, and that includes facility for selective testing of system operation independently of actual material level and independently of transducer condition. Another object of the present invention is to provide an apparatus of the described character that is self-calibrating in the sense that changes in temperature, material characteristics, and enclosure style or material are automatically accommodated in both the measurement and self-test modes--i.e., without operator intervention or adjustment.
In the apparatus in accordance with the preferred embodiment of the invention, electrical signals are applied to the transmitting crystal to radiate ultrasonic energy at different high and low frequencies into the material space toward the receiving crystal. Electronic circuitry is responsive to the energy received at the receiving transducer at high frequency for indicating presence of material in the space between the transducers, and at the lower frequency for indicating operative condition of the apparatus. At high frequency, the energy is radiated into the material gap, and energy at the receiving crystal thus indicates presence of material in the gap. At lower frequency, the entire structure of the apparatus is placed in resonance, and energy at the receiving crystal indicates proper operation of the system.
In the preferred embodiment, the transmitting crystal is coupled to a swept frequency oscillator for continuously sweeping back and forth between the high and low ultrasonic frequency ranges. This technique has the advantages not only of automatically and continuously testing system integrity at low frequency between each high frequency measurement cycle, but also readily accommodates a wide variety of differing measurement conditions, liquid temperatures and densities, air bubbles or the like, which would otherwise affect measurement reliability if specific fixed measurement and test frequencies were employed. That is, resonant frequency of the apparatus can change with enclosure and probe geometry, enclosure and probe composition, geometry and composition of the vessel to which the enclosure and probe are mounted, as well as presence or absence of material in the vessel. In the same way, optimum measurement frequency can change with material composition and temperature, presence of air bubbles, etc. as discussed above. By continuously sweeping back and forth between high and low frequency ranges during operation, such changes in optimum self-test and measurement frequencies are automatically accommodated. The electronics may therefore be employed in conjunction with a number of probe and enclosure geometries and compositions, and the entire system may be employed in conjunction with a wide variety of vessels and materials, without design change or adjustment. Furthermore, once installed in a particular application, any changes due to variations in operating conditions are effectively ignored.
In the preferred embodiment of the invention in which the energy radiated into the material space is continuously and alternately swept back and forth between high and low frequency ranges, the energy received at the receiving crystal transducer is converted to a pulsed signal having a pulse width that varies as a function of time during which ultrasonic energy is received at the receiving transducer. If only air is present between the transducers, this pulse duration would be relatively short, reflecting duration of resonance in the low frequency range. If material is present between the transducers, the pulsed signal would be of longer duration, while a system failure would result in a signal of zero pulse duration. Time duration of the pulsed signal is therefore compared to a first threshold for indicating system failure when the pulse duration is below such threshold, and to a second higher threshold for indicating presence of material between the transducers when the pulse duration is of correspondingly greater duration. Separation of these thresholds accommodates a variety of differing materials and measurement conditions as described above.
The apparatus of the preferred embodiment of the invention also includes facility for selectively testing operation of the apparatus independently of the transducers and of presence or absence of material in the space between the transducers. Specifically, a pair of switch elements are connected in series between the transducers. The first switch element is normally open, and closes in response to selection of a test mode of operation. The first switch device may comprise a reed switch responsive to placement of a magnet externally adjacent to the apparatus, or a photo-optical switch responsive for remote selection of a test mode of operation. The second switch element, which may comprise jumpers programmed at the time of manufacture or installation of the apparatus, has one conductive condition for applying energy from the first switch, when closed, to the second crystal, and thereby simulating presence of material between the transducer crystals independently of actual material level. In the second condition of the second switch element, energy from the first crystal through the first switch element is fed to ground, thereby simulating system failure.